Wireless access systems are use in automotive technology to unlock a motor vehicle or to make it possible to open a vehicle door. With passive wireless access systems a person carries an ID (identity) transmitter with them. This ID transmitter communicates with a base station located in the vehicle as soon as it comes closer than a specific distance to the vehicle or the base station. With passive systems the base station usually emits a signal in the long-wave range, which is also referred to as Low Frequency (LF). The transmit frequency is around 125 kHz or 135 kHz for example. The ID transmitter, also referred to as a transponder below, responds to the base station with a signal in the decimeter or Ultra High Frequency (UHF) wave range. In Europe this frequency is typically 432 MHz or 868 MHz. The transponder transmits at the high frequency since transmitting at a low frequency over the relatively long distance requires a lot of energy, which the battery of the ID transmitter cannot provide over the long term.
U.S. Pat. No. 7,317,376 B2 discloses a combined system of an access system and an immobilizer. An immobilizer ensures that the automobile can only be started when the driver is in the vehicle. With the immobilizer presented in U.S. Pat. No. 7,317,376 B2, the base station sends out a low-frequency signal to which the transponder likewise reacts with a low-frequency signal. For the short distances between transponder and base station the transmitter can also emit the low-frequency signal with little energy. In this device the low-frequency signals for the immobilizer and the access system are controlled with the same antenna, which is disadvantageous however since the requirements for the signal strength and the quality of the resonant circuit for the two systems differ and the transmit powers cannot be jointly well optimized. If different antennas are used for the two systems, the problem arises however that the outlay for the systems increases.